The present invention relates to a process for the production of propylene block copolymers. More particularly, it relates to a process for producing with high efficiency propylene block copolymers which are superior in physical properties, particularly in impact resistance and stiffness.
Crystalline polypropylene is superior in, for example, stiffness, heat resistance and surface gloss, but has a disadvantage in that its impact resistance is poor.
In order to overcome the disadvantage of poor impact resistance in the crystalline polypropylene, various methods in which propylene block copolymers containing an ethylene unit are produced have been proposed. One of the methods involves forming a propylene homopolymer at the first step, further polymerizing propylene at higher temperatures than in the first step at the second step, and then copolymerizing ethylene and propylene at the third step (see Japanese patent application Laid-Open No. 71712/1980). In accordance with these conventional methods, however, the productivity of the desired polymers drops and there cannot be produced copolymers which are well balanced in impact resistance and stiffness.